German chemists have developed two sustainable plastic alternatives to high-density polyethylene that can be chemically recycled more easily and nearly 10 times as efficiently, thanks to “break points” engineered into their molecular structures.
Derived from plant oils, the new plastics were presented in a paper published Wednesday in Nature as low-waste, environmentally friendly replacements to the conventional fossil fuel-based plastics that enter natural ecosystems at a rate of millions of tons per year.
Most recycling performed today is mechanical recycling, in which plastic is sorted and sliced into pellets that are then used to create new plastic materials. Chemical recycling, in contrast, involves breaking down the long polymer chains of plastic with heat or solvents to retrieve the material's initial monomer components.
One of the obstacles to developing chemical-recycling technology is also a reason why plastic is a useful material: the strong carbon-carbon bonds in its molecular structure. Polyethylene, the most common kind of plastic, requires at least 600 degrees Celsius to break those bonds to retrieve the monomers, and is chemically recycled at a rate lower than 10%.
“Stability of the hydrocarbon chains is rather a problem in that case,” said Stefan Mecking, the lead author of the study and the department chair of chemical materials science at the University of Konstanz in Germany. “To really break them down into small molecules needs high temperatures and is energy intensive, and also the yields are not that good.”
The plastic compounds created by Mecking and his colleagues had chemical bonds that could be more easily broken so chemically recycling them would be more effective.
Chemically recycling the two materials, which were forms of polyester and polycarbonate, required placing them in ethanol or methanol with a catalyst at only 120 degrees Celsius, or 150 degrees without the catalyst. The researchers then cooled and recrystallized the plastic before filtering it out. In the case of the polycarbonate, 96% of the initial material was recovered.
As demand in the recycling industry stalls and recyclable materials pile up in warehouses and landfills, chemical recycling has been offered as part of a solution to reduce plastic waste. The industry group American Chemistry Council has praised the emerging technology for the significant role it could play in a circular economy by reducing plastic waste and repurposing it into new products.
Yet the method is not without pushback. A 2020 report by the environmental organization Global Alliance for Incinerator Alternatives criticized a lack of research and reporting in current chemical-recycling technologies and said the process is energy-intensive and may emit hazardous chemicals. Chemical recycling also distracts from efforts to limit plastic production to reduce waste, the organization said.
In the new research, the chemists found that the recycling process worked when the plastic contained dye or fillers such as carbon fibers, both of which cause challenges in mechanical recycling. The plastics were also successfully recovered when pieces of other plastics were included in the alcohol solvent.
Plant oils were chosen as starting materials for synthesizing the plastics primarily because of their useful long chains. They are also more sustainably sourced than the crude oil and other fossil fuels used to produce most of the world’s plastic material.
The new plastics were very similar to high-density polyethylene, the widely used plastic labeled as recycling number 2. Testing by the chemists found that the three materials had comparable properties such as structure, elasticity and molecular weights — though the polycarbonate and polyester had lower melting and crystallization points.
They were also better suited for 3D printing than polyethylene, and they retained their properties after recycling and reuse.
The one disadvantage of the new materials Mecking identified was their cost. Ethylene is the “cheapest building block of the chemical industry,” he said, so, "Competing with conventional polyethylene at the current market and legal framework conditions is very difficult.”
Mecking and his colleagues are conducting ongoing research into using their new plastics for 3D printing, an initial application he said would be exciting for further developing the material and eventually scaling up its production.
These new plastics may also biodegrade in nature more quickly than common polymers because of their engineered break points, a line of inquiry that also interests the German chemist.
The article, “Closed-loop recycling of polyethylene-like materials,” was published Feb. 17 in Nature. The authors of the study were Manuel Häussler, Marcel Eck, Dario Rothauer and Stefan Mecking, University of Konstanz. The lead author was Stefan Mecking.